Anaerobic fermentations of hydrogen and carbon monoxide involve the contact of the substrate gas in an aqueous fermentation menstruum with microorganisms capable of generating alcohols such as ethanol, propanol, i-butanol and n-butanol. The production of these alcohols requires significant amounts of hydrogen and carbon monoxide. For instance, the theoretical equations for the conversion of carbon monoxide and hydrogen to ethanol are:6CO+3H2O.C2H5OH+4CO2 6H2+2CO2.C2H5OH+3H2O.
As can be seen, the conversion of carbon monoxide results in the generation of carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion. For purposes herein, it is referred to as the hydrogen conversion.
Typically the substrate gas for carbon monoxide and hydrogen conversions is, or is derived from, a synthesis gas (syngas) from the gasification of carbonaceous materials, from partial oxidation or reforming of natural gas and/or biogas from anaerobic digestion or landfill gas or off-gas streams of various industrial methods such as off gas from coal coking and steel manufacture. The substrate gas contains carbon monoxide, hydrogen, and carbon dioxide and usually contains other components such as water vapor, nitrogen, methane, ammonia, hydrogen sulfide and the like. (For purposes herein, all gas compositions are reported on a dry basis unless otherwise stated or clear from the context.)
These substrate gases are typically more expensive than equivalent heat content amounts of fossil fuels. Hence, a desire exists to use these gases efficiently to make higher value products. The financial viability of any conversion process, especially to commodity chemicals such as ethanol, will depend, in part, upon the costs of the feedstocks, conversion efficiency and operating and capital costs for generating the substrate gases; and upon the capital costs, the efficiency of conversion of the carbon monoxide and hydrogen to the sought products and the energy costs to effect the conversion of the substrate gases to the higher value products.
In a bioreactor, hydrogen and carbon oxides pass from the gas phase to being dissolved in the aqueous menstruum, and then the dissolved hydrogen and carbon oxides contact the microorganisms for bioconversion. Due to the low solubilities of carbon monoxide and, especially, hydrogen in aqueous media, mass transfer can be a factor limiting rate and conversion in the bioconversion to alcohol. Therefore challenges exist in the design of commercial scale bioreactors that provide for the sought mass transfer while still enabling a high conversion of gas substrate at capital and operating costs that enable such a facility to be commercially competitive.
From the standpoint of low capital and energy consumption, deep tank bioreactors have been proposed to provide longer contact times between the substrate gases and the aqueous fermentation menstruum with the objective of obtaining higher conversions of the substrate gases to the higher value products. In deep tank bioreactors, the height of the aqueous menstruum is a significant determinant of the contact time for the mass transfer and bioconversion to occur. On a commercial scale, deep tank bioreactors have a depth of at least about 10, preferably at least about 15, meters.
One type of deep tank bioreactor is a stirred tank bioreactor which uses a motor driven impeller to provide liquid flow in the bioreactor and distribute the gases in the aqueous menstruum. The stirring may also facilitate increasing the contact time between the gases and the aqueous menstruum. Due to the scale, low stirring rates are typically used in deep tank bioreactors. Another type of deep tank bioreactor is a bubble column bioreactor wherein the substrate gases are introduced at the bottom of the vessel and bubble through the aqueous menstruum. Advantageously, commercial-scale bubble column bioreactors are relatively simple in design and construction and require relatively little energy to operate. Achieving liquid mixing in a deep tank bubble column can be problematic. Mechanically pumping aqueous menstruum may facilitate liquid flow. As discussed herein, the use of smaller bubbles may form lower density dispersions that facilitate mixing. Moreover, smaller bubbles favor the mass transfer of hydrogen and carbon oxides from the gas phase to liquid phase. A third type of deep tank bioreactor uses one or more, gas-lift riser sections to facilitate liquid flow and mixing. Typically, gas is introduced at the bottom of a riser section and due to a lower density, the aqueous menstruum flows upwardly. At the top of the riser section, the liquid phase passes to a down flow section for return to the bottom of the riser section.
The off gases from bioreactors contain substrate that was not bioconverted and diluents such as methane and nitrogen. Although off gases can be recycled to the bioreactor or passed to another bioreactor, challenges can exist. For instance, the substrate gases may contain diluents that if recycled to a bioreactor, can build-up and reduce the partial pressure, thus reducing the driving forces for mass transfer of hydrogen and carbon monoxide to the aqueous menstruum. Moreover, the off gas from a deep tank bioreactor would need to be compressed for recycle or for passage to a sequential bioreactor. A sequential bioreactor represents additional capital and operating costs, and since the concentration of hydrogen and carbon monoxide in the off gas from the firstbioreactor is reduced due to the anaerobic bioconversion, the incremental conversion efficiencies achieved may not be economically justifiable.
Bell in United States published patent application No. 20100105118 discloses an integrated process for making alcohols which is said to provide high bioconversions of carbon monoxide in fermentations in the absence of oxygen. Bell notes at paragraph 0013 that in theory, carbon dioxide may be used as a reactant for the production of higher alcohols such as ethanol. However, he states that in practice the fermentation route to higher alcohols tends to be a net producer of carbon dioxide. In his disclosed process, the gas from the bioreactor which contains carbon dioxide is fed to a reaction section of a steam reformer. The reformer is either operated dry or with a mole ratio of water to carbon dioxide of less than 5:1. Bell states in paragraph 0025:                “ . . . the integrated process of the present invention operates with a hydrogen excess and efficiently converts the carbon dioxide in the feed to the reforming process to carbon monoxide, and actually results in a lower process inventory of carbon dioxide.”Bell confirms the carbon dioxide net make of his process and the low conversion of hydrogen in the examples. In Example 1, 107 kmoles per hour of carbon dioxide are fed to the bioreactor and 194 kmoles of carbon dioxide are contained in the off gas from the bioreactor. Hydrogen is fed to the bioreactor at a rate of 318 kmoles per hour, and 231 kmoles per hour of hydrogen are contained in the off gas for a hydrogen conversion of about 28 percent. Similarly in Example 2, the feed to the bioreactor contains 25 kmole per hour of carbon dioxide, and 117 kmole per hour of carbon dioxide is contained in the off gas. Hydrogen is fed to the bioreactor at a rate of 298 kmole per hour with 206 kmole per hour of hydrogen passing to the off gas for a hydrogen conversion of about 31 percent. Bell subjects the off gas to a membrane separation unit operation to remove hydrogen to reduce the amount of hydrogen being passed back to the reformer. This hydrogen is fed to the hot box of the reformer as a portion of the fuel. See paragraphs 0074 and 0075.        
Although Bell may have reduced carbon dioxide emissions as compared to the use of autothermal reforming or traditional steam reforming, the low conversion of hydrogen detracts from the commercial viability of the disclosed process.
Processes are therefore sought that can provide very high conversions of hydrogen and carbon monoxide in commercial-scale, continuous operations to alcohol.